The present invention relates to sensing apparatus employing optical fibres, and in particular, although not exclusively, to sensing apparatus incorporating optical fibre sensors mounted in catheter probes for medical applications such as in-vivo measurement of pressure.
Sensors for particular measurands, such as pressure, temperature, and strain, are often required to be as small as possible, and this is particularly true for in-vivo medical applications. Sensors are also often required to be passive, ie. requiring no electrical power input in order to function, or to dissipate negligible or zero power during operation.
Optical sensors are good candidates for applications having these requirements, and indeed optical sensors that exploit optical fibre technology are most attractive devices for application in medical procedures associated with diagnosis and intervention. They have a number of important advantages over more classical sensors (eg. electronic), namely, their small size, immunity to EM-noise, high degree of bio-compatibility, high sensitivity, ease of sterilisation, and passive operation. However they often necessitate the use of complex and expensive optics and electronics as is the case with interferometric based optical fibre sensors. It is therefore desirable to reduce the complexity and cost of fibre optic sensing apparatus.
Fibre optic sensors based on numerous principles of operation are well-known, including those based on interferometry, non-linear effects, fluorescence (e.g. as a function of temperature), dimensional changes of in-fibre Bragg gratings, and amplitude modulation of light signals. Both extrinsic and intrinsic amplitude modulation sensors are known. In extrinsic amplitude sensors, light exits from an optical fibre and the sensor is configured such that a varying amount of light is recaptured in another or the same fibre, the amount being dependent on the particular measurand. Some of the light input to the device is therefore lost, reducing the power of the recaptured signal. Furthermore, the power of the recaptured signal, rather than being a function of the measurand only, is instead dependent on the input light power, which may vary.
Intrinsic amplitude sensors have typically involved the measurand interacting with an optical fibre and leading to a variation in light loss in the fibre. Such interaction usually takes the form of squeezing or flexing the fibre such that micro bending loss occurs, and again input light power is lost and the output is affected by fluctuations in the input light power.
Any fibre optic sensors which rely on the intensity of the output signal have the inherent disadvantage of being sensitive to variations in the power level of the light source.
It is desirable, therefore, to provide sensing apparatus and a measurement method which address the problems associated with the prior art.
According to a first aspect of the present invention there is provided Sensing apparatus including:
a sensor comprising
a fused tapered fibre optic coupler formed of two optical fibres fused together to provide a fused portion which is drawn down to form a taper waist portion, the coupler having an input end comprising an input unfused portion of one of said two optical fibres and an output end comprising an output unfused portion of one of said two optical fibres;
a light source arranged to input light to the taper waist portion along the input unfused portion; and a light detector arranged to generate a signal indicative of a parameter of the light transmitted to the output unfused portion from the taper waist portion,
characterised in that the taper waist portion is formed as a loop and at least part of the loop is arranged to bend in response to a measurand.
The sensor may further comprise bending means arranged to bend at least part of said loop according to the measurand.
The parameter of the light, of which the generated signal is indicative, may for example be the light power, intensity, or wavelength.
The fused tapered coupler may be a typical 2xc3x972 device comprising two unfused input portions and two unfused output portions, or may be formed from three or more optical fibres.
Alternatively, the fused tapered coupler may have only one unfused input portion. Such an arrangement may be formed by cutting off or otherwise removing one of the input portions of a 2xc3x972 coupler, or by suitable coupler fabrication.
Similarly, the output end may comprise a single infused output portion, or two or more unfused output portions.
The light source may be a simple light source, such as a LED. The taper waist portion typically has a substantially uniform cross sectional area along its length, and the drawing down process, which, in the art, is also referred to as xe2x80x9ctaperingxe2x80x9d or xe2x80x9celongationxe2x80x9d or xe2x80x9cpullingxe2x80x9d, results in that cross sectional area being smaller than the sum of the cross sectional areas of the unfused fibres. Fusing together and drawing down (i.e. pulling in a controlled fashion) the optical fibres enables optical interaction between them. Thus, although in certain embodiments light is input to the sensor along only one unfused xe2x80x9cinputxe2x80x9d fibre, in general not all of the light reaching the output end will be transmitted to one output unfused portion. In embodiments where the output end comprises two unfused output portions, in general the total light power emerging from the device along the output fibres will be shared between them. A splitting ratio may be defined as the ratio of the light powers propagating in the two unfused output fibres, but is often defined in terms of the light power in one output fibre expressed as a fraction or is percentage of the total emerging power.
The optical field within the tapered portion is very sensitive to changes in geometry, and bending the loop will, in general, result in a change in the splitting ratio. The term xe2x80x9cbendingxe2x80x9d is used to denote any action resulting in deformation, deflection, distortion or change in the curvature of the loop, in part or as a whole.
In embodiments where the output end comprises a single unfused portion, deformation of the loop, in general, results in a change in a parameter of the light transmitted to the output portion, for example a change in its intensity.
The sensor is arranged so that the bend applied to the loop is in accordance with the quantity being measured by the sensing apparatus, i.e. the measurand. Thus the applied bend is a substantially reproducible function of the measurand. For example, the end of the loop may be deflected sideways by a distance proportional to the magnitude of the measurand.
Changes in the measurand result in changes in the bend applied to the loop, and hence changes in the splitting ratio. This in turn leads to a change in the signal generated by the light detector, which can therefore be used to monitor the measurand.
This first aspect of the present invention provides numerous advantages including:
a) By arranging the taper waist portion as a loop, the sensor can have a probe-like form, with input and output fibres at the same xe2x80x9cendxe2x80x9d of the loop;
b) An indication of the measurand can be obtained by simply monitoring the magnitude or other aspect of the signal from the light detector, using for example a photodiode. The sensing apparatus may thus have low complexity and cost;
c) As the parameter of the light transmitted to the output unfused portion (or the splitting ratio) is very sensitive to changes in the geometry of the taper waist region, the bending means may be engineered in a wide variety of ways, to suit particular applications. Providing that the bend applied to the loop is in accordance with the measurand, i.e. a substantially reproducible function of the measurand, then the generated signal will be a useful indication of the measurand. Thus, there is considerable design freedom. In addition, reproducable bending may be easier to engineer than the application of positive axial strain. Also, the loop need not be encapsulated in a holding medium. This produces the advantage that light propagating in the taper waist portion may be strongly confined, reducing losses;
d) The sensing apparatus comprises a passive sensor. In effect, the sensor is an intrinsic amplitude modulated device, but advantageously does not modulate amplitude by varying loss. Instead, the light transmitted to the output fibre (or the splitting ratio) is modulated, and the sensor can thus deliver a stronger output signal.
Advantageously, the loop may have been annealed after its formation. In the annealing process the loop is heated (typically by a flame) sufficiently for stresses caused by the bending of the taper waist portion to form the loop to be relaxed, but without causing significant additional loss of output power. Annealing can improve the stability of the loop and/or its mechanical reliability.
The loop may be nominally planar (i.e. it is formed in a plane and/or it lies substantially in a plane when no force is applied to it by the bending means) and the bending means may be arranged to bend the loop substantially in or out of this nominal plane.
Advantageously, the output end may comprise two unfused output portions of optical fibre and the sensing apparatus may comprise a second light detector, the two light detectors being arranged to generate respective signals indicative of parameters of the input light transferred to each of the output unfused portions (i.e. the nominal output fibres). The signals may then be combined in such a way so as to provide an output signal that is indicative of the measurand and is independent of the input power. For example, the output signal may vary in proportion to the ratio of the respective signals or the ratio of the difference between the signals to their sum.
According to an embodiment of the present invention there is provided sensing apparatus including:
a sensor comprising
a taper waist portion of optical fibre formed by fusing together and drawing down respective portions of two optical fibres,
a first taper transition portion of optical fibre at a first end of the taper waist portion, optically connecting the taper waist portion to a first two respective unfused portions of said two optical fibres and being a portion over which a transition from the taper waist portion to said first two respective unfused portions occurs, and
a second taper transition portion of optical fibre at a second end of the taper waist portion, optically connecting the taper waist portion to a second two respective unfused portions of said two optical fibres and being a portion over which a transition from the taper waist portion to said second two respective unfused portions occurs, the apparatus further including
a light source arranged to input light to the taper waist portion along one of said first two respective unfused portions; and
a light detector arranged to generate a signal indicative of a parameter of the light transmitted to one of said second two respective unfused portions from the taper waist portion,
characterised in that the taper waist portion is arranged as a loop and the sensor further comprises bending means arranged to bend at least part of said loop according to a measurand.
Conveniently, the optical fibre portions of embodiments of the present invention may be provided by a pre-formed fused tapered 2xc3x972 bi-directional coupler. These devices are well-known and are formed by holding in contact and stretching and fusing along a section two optical fibres in a heat source such that optical interaction between the fibres becomes possible. Typically, these devices are fixed and packaged such that the fibres are held taut. A 2xc3x972 device has two input and two output fibres, and xe2x80x9cbi directionalxe2x80x9d indicates that the roles of the nominal inputs and outputs can be interchanged. Clearly, fused tapered couplers having other numbers of input and/or output fibres may be used in embodiments of the present invention. A schematic diagram of a known fused tapered coupler is shown in FIG. 1. The taper waist portion 1 has a reduced cross sectional area. The taper waist is also known as the taper neck. Taper transition portions 2,3 optically connect the taper waist 1 to unfused portions 21,22 and 31,32 of the input and output fibres respectively. These devices are also known as fused bi-conical tapered couplers as the taper transition portions are substantially conical. It is known to control the fusing and drawing down (tapering) process to give a desired taper transition portion profile. A linear taper is known to be stiffer, i.e. mechanically more resistant to bending, than exponential transitions.
Advantageously, the optical fibres may be single mode (also known as mono mode) fibres. Such fibres comprise a core surrounded by a sheath of cladding material having a lower refractive index (n) than the core. The core is typically circular with a sufficiently small diameter such that only the fundamental mode can propagate down the untapered fibre. This fundamental mode is guided in the untapered fibre by the core-cladding boundary. The core diameter is typically smaller than 15 xcexcm but other sizes are also known. By employing single mode fibres and inputting light down only one of the unfused input fibres, more pronounced variations in splitting ratio with bending can be achieved than with multimode fibres. The taper waist portion may be drawn down to such an extent that the core materialxe2x80x94cladding material interface in the waist is no longer practically able to confine and guide the fundamental mode. In this situation, the fundamental mode is strongly guided by the cladding material external boundary (typically the interface with air) as it propagates down the taper waist, and the cores no longer play a role. Initially, the fundamental mode propagates along the input fibre (unfused) guided by the fibre core. On entering the first taper transition section it sees a core of gradually reducing radius. There comes a point where the core is too small to guide the fundamental mode, which then xe2x80x9cbreaks outxe2x80x9d, to be guided by the claddingxe2x80x94air interface, i.e. the propagating field is now over the entire waist cross section.
It is known that a sufficiently tapered region of a single mono mode fibre is less prone to bend loss than the untapered fibre because the fundamental mode, is previously weakly confined by the core, is strongly confined in the tapered region by the claddingxe2x80x94air boundary. For example, in the paper xe2x80x9cMiniature High Performance Loop Reflectorxe2x80x9d, Oakley et al, Electronics Letters Dec. 5th, 1991 Vol. 27 No 25 pp 2334-2335, it is reported that a 1.5 mm diameter bend can be formed without introducing measurable loss (i.e. in this case less 0.05 dB) in a tapered waist region of a single mode fibre, the untapered fibre having a core diameter of 10 xcexcm, a cladding diameter of 125 xcexcm, and a cut-off wave length of 1250 nm, and the cladding diameter in the taper waist originally reported as being 30 xcexcm. The true cladding diameter in the taper waist was in fact 15 xcexcm, as was reported in a correction published later. In contrast, the minimum bend diameter of the untapered fibre consistent with low loss was approximately 4 cm.
It has been determined that in embodiments of the present invention, by drawing down the optical fibres sufficiently to ensure detachment of the input fundamental mode field from the input fibre core in the taper transition region, the loop in the taper waist portion can incorporate a sharp bend with negligible additional loss. Advantageously, the taper waist portion may have a diameter of less than 50 xcexcm.
Preferably, the taper waist portion may have a xe2x80x9cdiameterxe2x80x9d of 30 cm or smaller. In general, the smaller the diameter of the taper waist the tighter the bend which can be made to form the loop without introducing unacceptable loss. However, the minimum diameter is determined by the wavelength of the light that the waist is intended to guide.
The loop may incorporate a bend having a diameter of 2 mm or less, and may be substantially, U-shaped, incorporating a 180xc2x0 bend. Advantageously for medical applications, the bend diameter may be 1 mm or smaller.
The size of the loop may therefore be reduced, to produce a compact sensor. Advantageously, the U-shaped loop enables the sensor to be arranged in a probe-like form. Sensors incorporating U-shaped loops with 180xc2x0 bends over diameters smaller than 1 mm are particularly suitable for medical applications and may be arranged inside catheters for in-vivo measurements.
Conveniently, the optical fibre portions of preferred embodiments of the present invention may be provided by fused tapered 2xc3x972 bi-directional couplers preformed from single mode fibres. Again, such devices are well-known, and an example is shown in FIG. 2. Each untapered fibre 21,22,31,32 comprises a core 211,221,311,321 surrounded by cladding 212,222,312,322. In the taper waist section 1 the cores have been reduced in cross section by the tapering process by such an extent that they no longer play a role in guiding light. The nominal positions of the respective cores in the taper waist portion are shown as broken lines 111,121.
Numerous models have been proposed for the mechanism by which light power input to only one of the input fibres 21,22 is shared between the two outputs 31,32. One of the most satisfactory explanations is as follows (see for example xe2x80x9cAnalyse d""un coupleur bidrectionnel a fibres optiques monomodes fusionneesxe2x80x9d, Bures et al, Applied Optics, Vol 22, No 12, Jun. 15, 1983 pp 1918-1922). The fused tapered waist portion 1 can be regarded as a single guide for the optical field, as the cores are too small in this region to play any part. The cladding of this single guide is the surrounding air. The fundamental mode propagating down one of the input fibres, say fibre 21, initially confined in the core 211, on entering the tapering transition region 2 sees a core which is reducing in size. There comes a point where this fundamental mode can no longer be confined by the core 211 and it xe2x80x9cbreaks outxe2x80x9d, to be confined now by the xe2x80x9csingle guidexe2x80x9d comprising the whole cladding material cross section in the taper waist 1. In effect, the single guide that is the taper waist is being excited_on only one side as a result of light being input along only one of the input fibres. A schematic cross section of the taper waist 1 along line A in FIG. 2 is shown in FIG. 3(a). This figures shows the excitation of one side of the single guide schematically, using an arrow to represent the electric field. This excitation of one side can be regarded as a superposition of the two lowest order modes of the single guidexe2x80x94the fundamental mode and an antisymmetric mode, as shown in FIG. 3(b). These two modes have different propagation constants (i.e. they propagate at different velocities along the single guide) and their superposition along the taper waist results in a beat pattern, the periodicity of which is determined by the difference between these propagations constants.
For a perfectly symmetrical coupler at certain positions along the taper waist the electric fields of the two modes will exactly cancel in one half of the single guide, and combine to give a maximum value in the other. Moving along the waist, the situation will then reverse. Thus, energy passes alternately from one side of the single guide to the other as we move along the waist. The splitting ratio of power in the output fibres 31,32 therefore depends on the position of the second taper transition region 3 with respect to the beat pattern, i.e. it depends on the distance the two modes have to travel along the single guide before the light field is recaptured by the cores 311,321 of the separate outputs. Thus, the splitting ratio is a function of the length of the taper waist portion 1.
Variations in splitting ratio can be achieved by applying positive axial strain to the taper region, and a sensor based on this principle is disclosed in the paper xe2x80x9cRatiometric fibre-optic sensor utilizing a fused biconically tapered couplerxe2x80x9d, Booysen et al, SPIE Vol. 1584 Fibre Optic and Laser Sensors IX (1991), pp 273-279.
For a perfectly symmetrical coupler (i.e. formed from essentially identical fibres) held straight, the above model explains the observed results well. The maximum splitting ratio (MSR) that can be achieved is 100% i.e. all of the output power may be in one fibre or the other. When the taper waist is bent, however, the model is difficult to apply. Also, when the taper waist is bent in the nominal plane of the fibres, there is now a structural distinction between the part of the single guide on the inside of the bend, and that on the outside. The MSR is no longer constrained by symmetry and can take any value. A MSR less than 100% means that it is not possible to have all of the output power in either of the output fibres. One of the fibres will be unable to carry more than a maximum amount, less than 100%, of the total output power. xe2x80x9cWavelength-flattened response in bent fibre couplersxe2x80x9d, O""Sullivan et al, Electronics Letters, Jul. 30th, 1992 Vol 28 No. 16 pp 1485-1486, describes the variation in MSR of a symmetric fused tapered coupler in singelmode fibre as a function of bend angle when the taper region is bent in the plane of the fibres. The taper waist region is initially straight and the purpose of the bending is to give the coupler a wavelength-flattened response.
A similar situation applies when the taper waist region has been formed from dissimilar optical fibres, for example, fibres of different diameters. In general, one of the output fibres will be unable to carry all of the output light.
These asymmetric effects may be thought of as resulting from unequal excitation of the two modes described above, i.e. as the two modes are launched down the taper waist, the input power is shared between them unequally.
Rather than being treated as a single guide, the taper waist may instead be considered as two separate guides which are affected differently by bending in their common plane.
In general, the dependence of the splitting ratio on loop or taper waist portion deformation, exploited by embodiments of the present invention, is a result of a combination of factors, which may include: the beating pattern of modes having different propagation constants; the symmetry of the taper waist; the orientation of the loop with respect to the taper waist symmetry; the direction or plane of bending; and a symmetry resulting in the unequal excitation of modes. Calculation of this dependence is likely to be complicated, and in practice the sensors of embodiments of the present invention will simply be calibrated against the particular measurand.
In preferred embodiments of the first aspect of the present invention, the loop may be nominally planar when no force is being applied to it by the bending means. The bending means may be arranged to bend the loop in its plane, or, alternatively, out of its plane. This may be achieved by applying a force to the tip of the loop, furthest from the taper transition portions, with the unfused portion of the sensor (e.g. the inputs and outputs) held fixed. Accurately reproducible deformation may therefore be achieved without the need for high complexity engineering, thereby facilitating design and improving design flexibility.
In embodiments where the taper waist portion has been formed from nominally identical fibres with circular cross sections, and as a result possesses two axes of substantial symmetry, the loop may be formed by bending the taper waist in the plane of the minor axis of symmetry, i.e. in the xe2x80x9ceasyxe2x80x9d direction. Here, xe2x80x9cminorxe2x80x9d is used to denote the axis of symmetry on which the taper cross section has the shorter projection. In such an embodiment, the bending means may be arranged to bend the loop out of its nominal plane, i.e. bending takes place parallel to the major axis of symmetry. This configuration provides the advantages that:
a) The loop is stiffer, i.e. more resistant to deformation, in this direction, which can enable more accurately reproducible measurements to be achieved;
b) The loop deformation is such that the portions of the taper waist corresponding to the constituent optical fibres are strained asymmetrically. This can result in more pronounced variation of splitting ratio for a given change in the measurand.
According to a second aspect of the present invention there is provided sensing apparatus including:
a sensor comprising
a taper waist portion of optical fibre formed by fusing together and drawing down respective portions of at least two optical fibres, the taper waist portion having a first end and a second end, and
a taper transition portion of optical fibre optically connecting the first end of the taper waist portion to at least one unfused portion of optical fibre, each unfused portion being an unfused portion of a respective one of said at least two optical fibres; and
a light source arranged to input light to the first end of the taper waist portion along one of said at least one unfused portions,
characterised in that the sensor further comprises
means for reflecting at least some of the input light propagating along the taper waist portion from the first to the second end back along the taper waist portion to the first end,
the sensing apparatus further comprises
a light detector arranged to generate a signal indicative of a parameter of the reflected input light transmitted to one of said at least one unfused portions from the first end of the taper waist portion, and
at least part of the taper waist portion is arranged to bend in response to a measurand.
Advantageously, the sensor may further comprise bending means arranged to bend at least part of the taper waist portion according to the measurand.
This second aspect provides all of the advantages listed for the first aspect, with, of course, references to xe2x80x9cthe loopxe2x80x9d now being replaced by xe2x80x9cthe taper waist portionxe2x80x9d.
As a result of light being input to and output from the same end of the taper waist region, the sensor conveniently may have a probe-like form.
In its simplest form the sensor may comprise just one unfused portion of optical fibre which is used for both input and output. Of course, in the taper waist region, optical coupling between the fused, tapered optical fibres and bending results in changes in a parameter of the light xe2x80x9crecapturedxe2x80x9d by the unfused portion after reflection.
Having only one input/output fibre provides the advantage that the size and complexity of the sensor are minimised, and the sensor may be incorporated in a probe of small diameter.
Such a sensor may be produced, for example, from a standard fused tapered coupler by removing one of the nominal input fibres.
Reflection of the input light may be achieved in a variety of ways. For example, in certain embodiments the sensor tapers out from the second end, connecting to unfused portions of the optical fibres, the ends of which are terminated at reflecting surfaces, or joined to provide longitudinal optical connection. Such sensors may be formed, for example,from a conventional fused tapered coupler, with the ends of the output fibres cleaved (to provide reflecting surfaces) or joined.
Alternatively, the reflection may be achieved by arranging the taper waist portion to terminate at the second end at a substantially planar surface, produced, for example, by cleaving (cutting). Again, such a sensor may be formed from a conventional 2xc3x972 fused tapered coupler, by cleaving the taper waist portion.
The taper transition portion may connect to two unfused portions of optical fibre (corresponding, for example, to the two nominal input fibres of a conventional coupler).
Light may thus be output from the sensor along a nominal input fibre of the coupler.
By employing a single pair of fibres as both inputs and outputs, and by reflecting light back from an end of the taper waist rather than looping it, the size and complexity of the sensor have been further reduced.
The reflecting end of the taper waist may have been produced by cleaving (cutting) and may be mirrored to increase the portion of input light reflected back along the taper waist. Reflection may, however, be achieved by other means, such as by looping and fusing the end of the taper waist back on itself.
In embodiments where the taper transition portion connects to two unfused portions,as with the first aspect, a second light detector may be provided, so that signals indicative of a parameter of the light transmitted to each unfused portion may be generated. An output signal dependent on the measurand but independent of the input power may be generated by suitable means.
Again, the optical fibre portions of the second aspect of the invention may conveniently be provided from a known fused tapered 2xc3x972 bi-directional coupler which may comprise monomode fibres as before. For use in apparatus according to this second aspect, these known devices may be cleaved at some point along the taper waist only one half of the cleaved structure is then required.
Advantageously, the length of the taper waist portion may be selected to give a desired stiffness.
The mechanisms by which bending of the taper waist portion alters the splitting ratio of light power transmitted back along the unfused portions of fibre are the same as those described above with reference to the first aspect.
Advantageously, the taper waist portion and the two unfused portions may be substantially co-planar, and the bending means may be arranged to deflect the taper waist portion in this plane, the unfused portions being held fixed. This plane will typically be parallel to the major axis of symmetry of the taper waist portion and so deflection in this plane will result in unequal straining of the two xe2x80x9chalvesxe2x80x9d of the taper waist portion, corresponding to the respective. constituent fibres. Again, this unequal straining can lead to more pronounced variation in splitting ratio for a given deflection.
In embodiments of either the first or second aspects, the taper waist portion may have a substantially uniform cross section, with at least a major axis of symmetry in a direction corresponding to a line joining the nominal centres of the constituent fibres during the fabrication process (fusing and drawing down). The cross section may be circular,eliptical, figure of eight resembling two overlapping circles,or any other shape depending on the shapes and sizes of the constituent fibres and the degree of fusing. Of course, the two fibres need not be the same shape or size.
Advantageously, the fibres may be fused together to such an extent that the taper waist portion has a substantially circular cross section. This is advantageous as a circular cross section is a more repeatable cross section than other geometries. Although there is no geometrical major or minor axis of symmetry, there is a functional major axis defined by the orientation of the transition region connecting the unfused portions to the waist.
Advantageously, the measuring apparatus may further comprise a tubular probe body, and the bending means may comprise a resilient membrane arranged to deflect according to a pressure difference between regions inside and outside the probe body. The sensor may be arranged inside the probe body, at one end, with the taper waist portion extending towards the end, and the input/output fibres running back along the probe. In this embodiment, the deflection of the membrane is communicated to the taper waist portion, which may in accordance with the first aspect be a loop, and the splitting ratio is modulated according to the pressure difference.
The membrane may seal an angled end of the tubular probe body, and may be in direct contact with the taper waist portion. The taper waist portion or loop may be arranged to lie nominally along the longitudinal axis of the probe. Advantageously, in this arrangement, deflection of the membrane bends the taper waist portion off the axis, i.e. out of its plane.
The probe body may comprise both flexible and rigid sections and may comprise a rigid section at or nearer to one end. The membrane may seal an orifice in the rigid portion. The probe body may be a catheter and may have an outer diameter of 1 mm or less.
According to a third aspect of the present invention there is provided a sensor comprising a fused tapered fibre optic coupler formed of two optical fibres fused together to provide a fused portion which is drawn down to form a taper waist portion, the coupler having an input end comprising an input unfused portion of one of said two optical fibres and an output end comprising an output unfused portion of one of said two optical fibres,
characterised in that the taper waist portion is formed as a loop.
The sensor may further comprise bending means arranged to bend at least part of the loop according to the measurand.
The body may comprise a substantially rigid section and the input and/or output portions may be attached to the rigid portion. The attachment may be substantially rigid, using for example epoxy resin or clamping means, or less rigid, using for example silicone rubber. The attachment means restricts the movement of the input and/or output unfused portions with respect to the rigid portion.
The bending means may be arranged to deflect the loop with respect to the rigid portion.
According to a fourth aspect of the present invention there is provided a sensor comprising
a taper waist portion of optical fibre formed by fusing together and drawing down respective portions of at least two optical fibres, the taper waist portion having a first end and a second end, and
a taper transition portion of optical fibre optically connecting the first end of the taper waist portion to at least one unfused portion of optical fibre, each unfused portion being an unfused portion of a respective one of said at least two optical fibres,
characterised in that the sensor further comprises means for reflecting light propagating along the taper waist portion from the first to the second end back along the taper waist portion to the first end.
The sensor may further comprise bending means arranged to bend at least part of the taper waist portion according to a measurand.
Again, the body may comprise a substantially rigid portion and one or more of the unfused portions may be attached to the rigid portion.
According to a fifth aspect of the present invention there is provided a measurement method comprising the steps of:
forming a loop from a fused taper waist portion of a fused tapered fibre optic coupler;
inputting light to the taper waist portion along a nominal input fibre of the coupler;
distorting said loop according a measurand;
generating a signal indicative of a parameter of the light transmitted to a nominal output fibre of the coupler from the taper waist portion; and
using said signal to provide an indication of the measurand.
Advantageously, the method may also include the step of annealing the loop.
According to a sixth aspect of the present invention there is provided a measurement method comprising the steps of:
inputting light to a taper waist portion of a fused tapered fibre optic coupler along a nominal input fibre of the coupler;
reflecting at least a portion of the light propagating along the taper waist portion from the input fibre back towards the input fibre;
generating a signal indicative of a parameter of the reflected light transmitted to a nominal input fibre of the coupler from the taper waist portion;
distorting at least part of the taper waist portion according to a measurand; and
using the signal to provide an indication of the measurand.
According to a seventh aspect of the present invention there is provided a measurement method comprising the steps of:
inputting light to a portion of optical fibre being optically connected at one end to at least two nominal output optical fibres, the inputted light being distributed non-uniformly over a cross section of said portion;
bending said portion according to a measurand;
generating a signal indicative of a ratio of respective light powers transferred to each of said nominal output optical fibres from said portion, and using said signal to monitor said measurand.
A non-uniform distribution of input light may be achieved by exciting only part of the cross section with optical field, for example by inputting light down only one of two input fibres fused and connected to the portion. However, the intensity of input light may be varied across the cross section in some other way, and may be non zero across the entire cross section.
In certain embodiments, light of different wave lengths may be input to different parts of the cross section. In general, any non-uniform distribution of input light over the cross section may be used which results in the splitting ratio being dependent on the geometry of the optical fibre.